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Over recent years there has been an increasing concern over global warming which is substantiated by a wealth of scientific. Global warming, or climate change as it is also know, is a grave and critical issue. The emissions of greenhouse gases, in particular carbon dioxide, are attributed as being the main caused of climate change. The Kyoto Protocol defines the six main greenhouse gases as being: carbon dioxide (COâ‚‚); methane (CHâ‚„); nitrous oxide (Nâ‚‚O); hydro fluorocarbons (HFCs); petro fluorocarbons (PFCs); and sulphur hexafluoride (SFâ‚†). Emissions of these greenhouse gases in the UK were 575 million tonnes. Carbon dioxide, the main greenhouse gas, made up over 80% of this at some 481 million tonnes.
The energy use in buildings accounted for nearly half of these carbon dioxide emissions with more than 25% coming from the energy we use to heat. Light and run our homes. With these stark sets of data the UK government published 'Building a Greener Future: Towards Zero Carbon Development'. It was within this publication that they confirmed their intentions for all new homes to be zero carbon b 2016
'We therefore believe we need to set a target now for moving to zero carbon housing within10 years. We would propose to achieve this in three steps: moving first, in 2010 to a 25% improvement in the energy/carbon performance set in building regulations; then second, in 2013, to a 44% improvement; then, finally, in 2016, to zero carbon. Zero carbon means that, over a year, the net carbon emissions from energy use in the home would be zero.'
Following the announcement of the UK governments target for zero net carbon emissions from new houses, house builders have had to consider ways in which they can reduce the carbon dioxide emissions from their new homes. There are a number of methods in which this can be contributed to including increasing insulation, designing buildings with regard to solar gain and designing in renewable technologies to provide space heating and domestic hot water heating.
'Renewables describe energy sources that do not deplete the earth's natural resources and do not create added waste products. They are therefore sustainable in that they can be used indefinitely without degrading the environment'
Renewable technologies represent an infinite source of energy and are abundant all around us in wind, water, sun and the earth. They can help us meet our energy demands without the use of fossil fuels, while causing minimal damage to the environment.
Although renewables are clearly a possible alternative to fossil fuel sources of energy, it is still unclear if it is economically viable to shift our economy to being fully dependant on renewable technologies. However, the zero carbon target means that the utilisation of renewables for our energy systems in domestic dwellings is fast becoming a necessity.
There are a number of renewable technologies currently available on the market for domestic builds, including:
Solar water heating
Solar Water Heating uses the radiation from the sun to heat water in a panel often sited on the roof which in turn can supply that heat as hot water or to a central heating system. If the system has been sized correctly, it can provide at least 40-60% of all your hot water requirements throughout the year
Micro combined heat and power
Micro-CHP is a specific form of CHP designed for individual households. As a replacement for a standard domestic gas boiler, it generates power mainly for consumption in the home alongside heat for space and water. Numerous types of Stirling engine, alongside fuel cell and Organic Rankine Cycle, micro-CHP units from a wide range of other manufacturers are currently in development or nearing commercialisation.
Wood fuelled heating
There are two main ways of using wood fuel to heat a domestic property; stand-alone stoves providing space heating for a single room or boilers connected to central heating and hot water systems. Stand-alone stoves can be fuelled by logs or pellets and are generally 5-11kW in output. The boilers are suitable for pellets, logs or chips and are generally larger than 15kW.
Air Source Heat Pumps
An air source heat pump extracts heat from the outside air in the same way that a fridge extracts heat from its inside. It can extract heat from the air even when the outside temperature is as low as minus 15â°C. Heat from the air is absorbed into a fluid which is pumped through a heat exchanger in the heat pump. Low-grade heat is the extracted by the refrigeration system and, after passing through the heat pump compressor, is concentrated into a higher temperature capable of heating water for the heating and hot water circuits of the house.
Ground Source Heat Pumps
A ground source heat pump circulates a mixture of water and antifreeze around a loop of pipe - called a ground loop - which is buried underground. Heat from the ground is absorbed into this fluid and is pumped through a heat exchanger in the heat pump.
Due to the zero net carbon emissions target for 2016 it is now imperative to consider alternative forms of fuel to fossil fuels in order to reduce the co2 emissions. A key consideration when choosing a suitable method of heating a house is the capital cost. This is both the installation cost and the cost in use of the system.
The installation of any of these renewable technologies will have a significant impact on the current carbon dioxide emissions produced from housing. In order to establish what forms or renewables are to be of interest to the general public is it necessary to examine our current energy consumption.
According to the National Statistics Publication 'Energy Consumption in the UK Domestic data tables', domestic energy consumption has increased by 18% between 1970 and 2009. However, since its peak in 2004, domestic energy consumption has fallen by 10% to 2009.
Space heating is responsible for the majority of domestic energy consumption. The proportion of total domestic energy consumption used for space heating has fallen from 60% in 1970 to 58% in 2008. However, the overall energy consumption for space heating has increased by 20% to 2008 from the 1970 level which is due to an increase in the number of households.
The above figure shows information provided by the Energy Saving Trust. As can be seen from the chart space heating is the major contributor to COâ‚‚ emissions. Almost half of the COâ‚‚ from homes in the UK are a result of pace heating. The second highest is lighting and appliances which include cold appliances such as fridges and freezers, wet appliances including washing machines and dishwashers, consumer electronics including televisions, DVD players and phones, Wi-Fi etc. The heating of water accounts for more than a fifth of domestic emissions with the rest being from cooking.
This chart clearly shows that space heating is the area where the greatest scope for reduction is possible. This, along with emissions from hot water heating, can be greatly reduced through increased efficiency in use and with alternative fuel sources to provide space heating and hot water heating.
There are a number of options available to provide heating to our homes with Gas and Electric Storage being the two most commonly used. In addition to these solid fuels, oil and renewable technologies may also provide heating.
The figure above shows the percentage of homes that have central heating installed by type as taken from the Energy Consumption in the UK Domestic data tables' 2010 update. In 2007 over 90% of households had central heating installed compared to just over 30% in 1970, this is due largely to a significant increase in the ownership of gas central heating.
* Assumed COâ‚‚ emission factors (SAP 2005):
Electricity - 0.422kg/kWh delivered
Gas - 0.194kg/kWh
LPG - 0.234kg/kWh
Oil - 0.265kg/kWh
COâ‚‚ emissions* and fuel use efficiency
(Domestic GSHP: Design and installation of closed-loop systems)
The figure shown above shows the efficiency of the four main heating fuels in the UK and their associated CO2 emissions. The figures show that Electricity emits 0.422 kg of CO2 for every kW of heat energy produced. Of the four fuels, Gas is the most efficient emitting just 0.194 kg of CO2 per kW of heat delivered and electricity being the most inefficient.
'As can be seen (assuming an average CO2 emission factor for electricity is 0.422kg/kWh) using a Ground Source Heat Pump with a seasonal efficiency of 350 percent would result in the emissions of 0.12kg of CO2 for every kWh of useful heat provided. By comparison a condensing gas boiler (assuming a CO2 emission factor for gas of 0.194kg/kWh), operating at a seasonal efficiency of 85 percent, would result in 0.23kg CO2 for every kWh of useful heat supplied'
(Domestic GSHP: Design and installation of closed-loop systems)
From this data it is clear that a Ground Source Heat Pump is more efficient than all four of the fuels considered above, even when compared to gas which is the most efficient of the four fuels. A Ground Source Heat Pump can reduce emissions by 50 - 60% of emissions compared with gas and a reduction of 70% - 80% over a direct electrical heating source. Therefore, a Ground Source Heat Pump is clearly considerably attractive in terms of energy efficiency.
The Ground Source Heat Pump
A heat pump removes the heat from one place, known as the 'heat source' and transfers it at a higher temperature to another place, known as the 'heat sink'. This works in a similar way to an air conditioning unit or refrigerator, but in reverse, as a refrigerator removes heat from inside and discharges it leaving the internal of the refrigerator, and its contents, cool.
A heat pump can be applied to a number of differing systems that utilise the ground, groundwater or surface water as a heat source. The most commonly installed systems are ground coupled systems, as there are a number of inherent disadvantages to using a water-coupled system including limited availability of suitable water sources and possibility of contamination of the water source.
There is a common misconception, to those not in the industry of renewable technologies, that ground source heat pumps take their heat energy from the earths core temperature. This is incorrect as the heat source of the soil is due to it absorbing the suns energy like a large solar collector.
'Ground source heat pumps make use of the energy stored in the earth's crust. Energy is transferred to and from the earth's surface by solar radiation, rainfall, wind etc. Only a small part (less than 3%) of the stored energy in the earth's crust comes from its core'
(Ground Source Heat Pumps - A Technology Review)
The amount of heat that is accessible is dependant on the location, geology and depth of the site to be considered. It is imperative to determine the depth of soil, the type of ground make up and the ground temperature. All these factors ineveitably affect the efficiency of the Ground Source Heat Pump as the temperature difference between the fluid in the 'heat exchanger' and the ground is what drives the heat transfer.
'At depths of less than 2m the ground temperature will show seasonal variations above and below the annual average air temperature. As the depth increases the seasonal swing in temperature is reduced and the maximum and minimum soil temperature begin to lag the temperature at the surface. At a depth of about 1.5m the time lag is approximately one month. Below 10 metres the ground temperature remains effectively constant at approximately the annual average air temperature (i.e between 10C and 14Cin the UK depending on the geology and soil conditions).'
This is illustrated in the graph below which shows the ground temperature as a function of depth for different periods of the year. Temperatures in the ground become more stable with increasing depth with the effects of seasonal variations only affecting the ground temperature to a depth of approximately 15 meters.
(BRE Report UKE-294)
Different types of ground have different thermal properties which can have a profound affect on the efficiency and performance of the Ground Source Heat Pump. It is therefore advisable to consider the thermal properties of the ground type at any site being considered for a Ground Source Heat Pump installation. Thermal conductivity (ks) and thermal capacity (cp) are the two main properties which have an affect on the design of the Ground Source Heat Pump.
How a Ground Source Heat Pump Works
A heat pump reduces the pressure of a liquid so that its evaporation temperature is lower. This evaporation process requires heat which is sourced from the ground in a Ground Source Heat Pump and the air in an Air Source Heat Pump.
When the vapour is compressed from a low pressure to a high pressure its boiling point is raised. This means that it wants to condense into a liquid again, which is achievable by releasing the heat it has absorbed. The 'heat sink' which can be the central heating system, domestic hot water heater or both is where this heat is transferred to.
The liquid is cooler than the heat source (the ground collector) and so flows naturally form the heat source to the evaporator. This causes the liquid to evaporate.
The vapour from the fluid then enters the compressor, which compresses the vapour increasing its temperature and pressure.
The high pressure vapour enters the condenser where it condenses at a higher temperature than the heat sink (central heating, domestic hot water or both) and as such heat flows naturally from the condenser to the heat sink (the ground collector)
The high pressure liquid enters the expansion valve where it reduces the pressure to its original point, and the cycle is complete and commences again.
(Getting warmer: a field trial of heat pumps)
As mentioned previously closed loop systems are the most commonly used in the UK but there are a number of differing configurations of Ground Source Heat Pumps. Three of these are closed loop systems and one is an open loop, a summary of which are detailed below:
Closed Loop - Most Ground Source Heat Pump installations in the UK are of the closed loop system like the one shown in the above diagram. The liquid, a mixture of water and antifreeze, is circulated through the 40mm buried polyethene pipe.
Horizontal Closed Loop - Require a large surface area and as such are best suited to properties with large plots of land. Large scale housing developers would be unlikely to provide sufficient plot space to install a horizontal loop. Horizontal pipes may be laid in single lengths or as a 'slinky', which comprises of overlapping coils of pipe and as such maximise the length of pipe laid per length of trench. These are buried at a depth of 2 meters and over.
Vertical Closed Loop 'Boreholes' - Are suitable for sites which have a smaller available plot area or where the soil layer is thin. This makes them suitable for use in towns and cities. Boreholes are typically 4 to 6 inches in diameter and vary in depth from 25 m - 200m deep. The disadvantage to using this system is that the drilling is more costly than excavation required typically required for a horizontal loop.
Surface Water Closed Loop - Can be installed if a site has an adequate body, of water at least 3m deep due to possibility of water freezing. A supply pipe is run from the heat sink to the water where it is coiled and laid along the bottom of the water. The need for only a small amount of excavation, to get the pipes to and from the pond, makes the installation the cheapest of the closed loop systems.
Open Loop - Water from a well or pond is used as the heat exchange fluid that circulated through the Ground Source Heat Pump system. Once it has passed through the heat exchanger, it is sent back to the source via a recharge well or surface discharge. An open loop system is the most inexpensive Ground Source Heat Pump system but does require regular cleaning and chemical inhibitors to prevent organic matters from contaminating the loop.
THE HEAT SINK
The Heat Sink is the place that the heat is delivered to and can be the central heating, hot water or both in a domestic building.
The central heating distribution system used with a Ground Source Heat Pump can vary and is dependant on the property itself, and in the case of retrofitting a system, the existing central heating that is installed
Most existing properties, and a number of new builds, in the UK have radiators installed as their heat distribution system.
The following table shows the supply temperatures required for a range of domestic heating distribution systems
Delivery Temperature (â°C)
30 - 40
Low temperature radiators
35 - 45
50 - 60
30 - 50
'Standard radiators are the cheapest option, and most retrofitted systems use the. Radiators require higher-temperature water, which would tend to make the heat pump work harder and thus achieve a lower Coefficient of Performance and system efficiency.'
(Getting Warmer: a field trial of heat pumps)
Ground Source Heat Pumps which are installed with conventional radiators do still offer an energy efficient option for central heating in comparison to electric or fossil fuels. However, due to the high temperature at which conventional radiators work it may be necessary to increase the thermal insulation of the building enabling the reduced heating requirement to be met using a reduced distribution temperature. Alternatively, "oversized" radiators, with an increased emitter surface of 30-40% can also be used successfully.
In buildings that have higher levels of thermal insulation, such as new build housing, there is a resultant low heating demand and as such low temperature air or water distribution systems of under floor heating may be used.
'Because underfloor heating works with water at 30â°C to 40â°C -as opposed to 50â°C to 60â°C in conventional radiators - it is considered a more energy-efficient option.'
(Getting Warmer; a field trial of heat pumps)
Domestic Hot Water
If the thermal capacity of the distribution system is too low the heat pump may suffer from prolonged periods when it is off due to light loads. As water heating provides a year-round load it can improve the load factor of the heat pump combating this problem. In domestic household hot water is usually required to be delivered from the tap at temperatures from 35C to 45C. As the heat pump will not be able to deliver the incoming water mains to this temperature a storage tank is required. The majority of domestic heat pumps have a maximum output temperature of 55C, and a maximum water storage temperature of 50C. At temperatures of 50C to 55C the potentially lethal bacteria Legionella may be present and as such an auxiliary electric heating immersion heater is needed to raise the water temperature periodically over 60C to kill the Legionella as pasteurisation occurs.
Ground Source heat pumps can be used to provide both central heating and domestic water heating, shown in a diagrammatical representation figure XXX. However, it must be noted that the heat pump will operate less efficiently when providing domestic water heating. This is due to the higher output temperature required. The auxiliary immersion heater as mentioned previously should not be able to operate at the same time as the heat pump is supplying heat to the hot water cylinder.
Condensing Gas Boiler
Condensing Oil Boiler
Air Source Heat Pump
Ground Source Heat Pump
Micro Combined Heat &Power
The table above shows the estimated cost of installing different heating systems. These prices are inclusive of the heat distribution system most suited to the heating system, for example, radiators with a condensing gas boiler, under floor heating with a ground source heat pump etc. The lower estimated price has been presented in the following chart.
The chart above shows that Ground Source Heat Pumps are the most expensive system to purchase followed by an Air Source Heat Pump, Micro CHP, Pellet boiler and then the fossil fuel systems of Gas and Oil.